1. Field of the Invention
The present invention relates to a control circuit for controlling an output voltage of a primary-controlled switched-mode power supply as well as to an associated method. The switched-mode power supply comprises a primary-side switch and a transformer with at least one auxiliary winding in which an auxiliary voltage is induced after opening the primary-side switch. The voltage induced in the at least one auxiliary winding provides the basis for the measurement voltage passed to the control circuit and for the supply voltage of the control circuit. The invention also relates to an associated switched-mode power supply.
2. Description of the Related Art
Normally, known switched-mode power supplies use a power transistor, for example an Insulated-Gate Bipolar Transistor (IGBT), as the primary-side switch to feed a pulsed current to a network of inductive and capacitive energy storage elements, which convert the switched current pulses into a controlled direct voltage. Switched-mode power supplies can supply output voltages which are greater, the same or of the opposite polarity to the uncontrolled input voltage, depending on the operating mode of the switched-mode power supply. Often switched-mode power supplies are used in power supply circuits, such as are required for example for a large number of electronic devices. Here, in particular in the case of mobile electronic devices, it is desirable that this type of switched-mode power supply accepts input voltages in the range of alternating voltage from 85 V to 270 V and therefore can be operated with different mains supplies anywhere in the world without modification or switches.
The output voltage of a switched-mode power supply is controlled by a feedback signal which replicates the output voltage. This feedback signal is used to control the working cycle of the switching power transistor. In order to provide a suitable feedback signal, various approaches exist; for example a primary-side auxiliary winding can be provided which, during the turn-off time of the primary-side transistor, generates a feedback signal which supplies a replica of the output voltage.
These types of switched-mode power supplies with auxiliary windings are for example shown in the German patent application DE 103 10 361, the European patent application EP 03 016 065.9, the U.S. Pat. No. 5,438,499 or the German published patent application DE 197 11 771 A1. In these cases, the signal generated in the auxiliary winding is passed to a feedback circuit which supplies the control signal to the control circuit. With a switched-mode power supply according to the flyback converter principle, with which the transferred energy per pulse remains constant and the duration of the spaces between the pulses is adjustable, as shown in EP 03 016 065.9, the output voltage can be very well replicated and controlled with the primary auxiliary voltage.
There is, however, the problem that the output current can only be acquired in a very complicated manner. For example, as shown in DE 103 10 361, the period of current flow in the secondary winding of the converter can be determined. Alternatively, an optocoupler can also be used as is shown, for example, in the European patent application EP 1 146 630 A2.
In order to be able to adjust the output voltage and the output current in a simple manner and to minimise the component costs necessary for this, a method of controlling the output voltage of a primary-controlled switched-mode power supply is proposed in which the switching frequency is adjusted in a linear relationship to the auxiliary voltage in that the switching frequency of the primary-side switch is determined by the charging time of a charging capacitor. This method is described in detail in the German patent application DE 10 2004 016927.6.
The switching arrangement of the control circuit from DE 10 2004 016927.6 is illustrated in FIG. 5. With the control circuit according to FIG. 5, the auxiliary voltage is measured on the supply voltage pin Vp of an application-specific integrated circuit (ASIC) 200. Therefore in a disadvantageous manner, the voltage level can only be set by changing a series resistance when generating the ASIC operating voltage. However, since this has a large influence on the trace of the output characteristic, trimming of the output voltage is only possible with restrictions. Furthermore, the circuit according to FIG. 5 has the disadvantage that the supply current of the ASIC 200 and the base current of the switching transistor 104 affect the voltage control, because they load the operating voltage of the ASIC 200, which is also the measurement voltage.
Moreover, as explained in detail in DE 10 2004 016927.6, the charging current of the capacitor Ct, which determines the time, is switched off as long as the measurement voltage lies above the reference value V Ct1. Therefore, with low load a long turn-off time must arise. However, since the discharge current of the operating voltage capacitor C2 is approximately constant, the reference voltage must be initially clearly exceeded to produce a sufficiently long discharge period. In contrast with a large load the turn-off time is short and the excess above the reference voltage is therefore only slight. Since the initial measurement voltage corresponds to the output voltage, the output voltage deviates corresponding to the excess from the set-point value and consequently is higher on open circuit than under load.
Furthermore, the solution proposed here has the disadvantage that the operating voltage cannot be rated such that the set output current also flows in the case of a short circuit; this is because during a short circuit the measurement voltage only reflects the voltage on the secondary diode and there is therefore insufficient voltage for the ASIC to operate correctly.
Finally, with the arrangement illustrated in FIG. 5 negative influences of the rectifying diode on the accuracy of control occur in the current control.
Overall, there are then three significant problems with a switched-mode power supply according to the arrangement of FIG. 5: On open circuit the output voltage is higher than under load; with a low input voltage the output current is lower than at nominal voltage; the output voltage cannot be trimmed without the characteristic trace changing.
The control circuit presented in DE 103 10 361 B4 offers improved control accuracy compared to DE 10 2004 016927.6. This circuit is however comparatively complicated and needs an eight-pin case which substantially increases the costs.